Makeshift camps holding thousands of refugees and migrants could appear in south-east England if the UK was to split from the EU, David Cameron is expected to claim. The prime minister, who is tipped to campaign against a Brexit ahead of the EU referendum, will apparently issue the warning to raise concerns about national security.

The Conservative leader will suggest that British border guards would no longer be allowed to check passports on French soil – a measure allowed under the 2003 Le Touquet Treaty – and the authorities would then be limited to inspecting the documents in the UK.

"The French would love to pull out of the arrangement," a 'senior government source' told The Daily Telegraph. "We will be telling people – look, if we leave the EU the Jungle camp in Calais will move to Folkestone. That is not something people want."

Read more: David Cameron's warning of a UK 'Calais Jungle' is using the public's immigration fears to sway EU vote

But Eurosceptics have accused the prime minister of "scaremongering" as he wraps up his renegotiation with Brussels.

Rob Oxley, a spokesman for the Vote Leave campaign, claimed Cameron's warning would be a "big mistake". He added: "Number 10 is in a blind panic over the failing renegotiation, hence Jungle scaremongering."

A day in the life of migrants in the 'Jungle' preparing to cross into BritainIBTimes UK

However, the pro-Brexit campaigns have been hit with infighting. Labour Leave were said to have split from Vote Leave after alleged internal arguments. John Mills, a major Labour donor who co-founded the group, later issued a statement on Friday 5 February to clarify that the group was not cutting its ties.

Meanwhile, Leave.EU co-founder and Ukip donor Arron Banks urged MPs to disaffiliate with Vote Leave and back the Grassroots Out campaign. The businessman also decided to attack Vote Leave's chief executive Matthew Elliott and campaign director Dominic Cummings in the letter.

Vote Leave and Leave.EU are competing to win the Electoral Commission's official Brexit campaign nomination, which will be decided once a date for the referendum is announced. Downing Street are reportedly hoping to hold the historic ballot on 23 June.

Despite the ongoing soap opera involving the Eurosceptic campaigns, "leave" is ahead in the opinion polls. The latest survey from YouGov for The Times, of more than 1,600 people between 3-4 February, put "leave" on 45% and "remain" on 36%.